Probe cards used in testing integrated circuits during their manufacture require periodic assessment and maintenance to avoid damaging the integrated circuits. As part of the assessment and maintenance of probe cards, optical inspection devices are used to measure the X, Y and Z positions of probes in a probe card in order to determine whether the probes are planar and whether they are properly aligned with the set pattern of bond pads on a semiconductor wafer that the probes are intended to address. The devices may also predict the length and location of a scrub mark that would be formed in a bond pad based on measurements made using a fiducial plate (i.e. checkplate) in lieu of a semiconductor wafer with a bond pad.
Even though each probe is very small, the force needed to achieve the appropriate amount of scrubbing is several grams. Where there are hundreds or thousands of probes in a single probe card, the amount of force needed to overtravel the probes to achieve a proper scrub mark can be very high. Often, overtravel of all the probes occurs at once. For example, the device may have a conductive checkplate that is used to measure the Z height of each separately wired probe and the Z height of the lowest of a group of bussed probes (some probes are wired together making it impossible to electrically separate the signals from each one). In this Z height measurement process, the checkplate is driven into contact with all of the probes and the Z height of the checkplate is recorded when each probe (or group of probes) makes electrical contact.
In order to measure the XY position of probes, also referred to as alignment of the probes, images of the probes may be captured. In some instances, probes are located within the image and linear encoders of the stages on which the camera is mounted are used to help determine the XY position. In other instances, the position of the probes is related in an image to fiducial marks that are formed on windows. Due to the large forces involved in overtravelling all of the probes into contact with the fiducial plate, deflections in the inspection system and the probe card can result. Thus, it is desirable to minimize the applied forces.
In addition to minimizing the amount of force applied to a probe card, it is desirable to minimize the amount of wear to which a probe card and its probes are subject. For example, in systems such as that described in U.S. Pat. Nos. 5,657,394 and 6,118,894 it is necessary for probes to touch down each time an optical alignment measurement is taken. Even considering that multiple probes may be imaged together, where a probe card has many hundreds or thousands of probes, each probe and the probe card will be subjected to much unnecessary wear that inevitably leads to a shorter life for a probe card and its probes.
Accordingly, there is a need for a mechanism for analyzing probe cards that minimizes stresses placed on a probe card and its probes and which similarly minimizes wear to a probe card and its probes.